The present invention relates to a method and a device for compensating undesirable side bands in nuclear magnetic resonance spectra.
For recording high-resolution spectra in nuclear resonance spectrometry (hereinafter called NMR spectrometry), the sample to be measured is placed in a DC magnetic field and subjected to rotational movement in order to average out a certain content of inhomogeneities of the DC magnetic field and to achieve thereby improved resolution of the spectral lines. However, this rotational movement gives rise in the spectrum to undesirable side bands, so-called rotational side bands. These appear on both sides of the respective real NMR spectral line, spaced by multiples of the frequency of the rotary movement.
The described rotational side bands are due, substantially, to three causes:
The inhomogeneities of the static magnetic field BO in the active sample volume, which are also known as BO inhomogeneities and which, together with the rotary movement of the sample, result in a modulation of the precession frequency of the nuclei. BO inhomogeneities may be caused by the field-generating magnet itself, but also by ferromagnetic, diamagnetic or paramagnetic materials in the neighborhood of the sample;
the inhomogeneities of the RF magnetic field B1, which are also known as B1 inhomogeneities and which are produced in the active sample volume during the transmitting phase by the transmitter and receiver coil, and which affect the nuclear signal induced in the receiver coil during the receiving phase; and
the non-ideal rotary movement of the sample which may be provoked, on the one hand, by the axis of symmetry of the sample being displaced and/or rotated relative the rotary axis and, on the other hand, by the fact that the sample cell does not present the ideal rotational-symmetry dimensions. Both these causes have the result, in particular, that the stray capacitance between the sample and the RF receiver coil varies periodically as a function of the frequency of rotation. Given the fact that this stray capacitance has to be added to the already very small capacitance of the receiving resonant circuit, this results in periodic detuning of the resonant circuit and, thus, phase modulation of the nuclear signal. In the case of a sample causing high-frequency losses this non-ideal rotary movement in addition may lead to periodic variations of the degree of attenuation of the receiving resonant circuit and, thus, to amplitude modulation of the nuclear signal. This phase and amplitude modulation, which is provoked by the non-ideal rotary movement of a sample, is called Q modulation. The latter is particularly disturbing in the case of high frequencies above 400 MHz where the receiving resonant circuit has a very low capacitance value only so that the influences of interfering capacitances make themselves felt very seriously.
In order to facilitate understanding, the phenomenon and the underlying causes of the rotational side bands will now be explained by way of FIGS. 1 and 2.
FIG. 1a shows a typical development of the main band A, with the associated rotational side bands B, B' and C, C' of a spectral line. The rotational side bands can be broken up into their respective components, based on the three causes described above.
FIG. 1b illustrates the components caused by the B-0 inhomogeneities. The side bands are symmetrical to the main band A, and are of like phase as the latter. This experimental finding is confirmed theoretically by a paper by H. Levitt published in "Journal of Magnetic Resonance 82", pages 427 to 433.
FIG. 1c illustrates the components caused by the before-mentioned B-1 inhomogeneities. According to the experimental findings, the side bands are asymmetrical relative to the main band A, are mostly spaced from the latter by multiples of the double frequency of rotation .omega..sub.m, due to the geometry of the receiving coil, and are again of like phase as the main band A.
FIG. 1d illustrates the side band components caused by the Q modulation. The side band components are antisymmetrical relative to the main band and may be of any phase relative to the main band. The antisymmetry is the result of the mainly capacitive periodic detuning of the receiving resonant circuit which principally results in a phase modulation of the nuclear signal.
FIG. 2 shows the evolution of an amplitude modulation AM and a phase modulation .phi.M of the nuclear signal K from a periodic detuning .DELTA..omega. of the receiving resonant circuit. The modulation frequency being small relative to the bandwidth of the resonant circuit, it is permissible to look at the situation in a quasi stationary way, as has been done in the case of FIG. 2. One obtains an amplitude modulation a(t) with twice the modulation frequency which, however, can be neglected in the case of small modulation deviations (FIG. 2a).
The phase modulation .phi.M illustrated in FIG. 2b, which occurs at the simple modulation frequency, is more disturbing.
In order to reduce these undesirable rotational side bands, one has proposed heretofore a number of different appliances which are intended to avoid the causes giving rise to the rotational side bands at the point of their generation.
One of these known measures consists in improving the homogeneity of the static magnetic field BO, for example by the use of very space-consuming and precise shim coils. However, these shim coil systems are not in a position to satisfactorily compensate the inhomogeneities of the static magnetic field resulting from the magnetic susceptibility of the RF receiving coil. This can be achieved only by proper selection of specific materials for the RF receiving coil having negligibly small susceptibility values.
In order to improve the B1 homogeneity, it has been proposed heretofore to optimize the geometry of the RF receiving coil for optimum field homogeneity.
However, the problem of Q modulation could not be overcome by these measures. As a result of the rise in field strengths of the static magnetic field encountered in the course of time in nuclear magnetic resonance spectrometry, the undesirable side band components (Q modulation) provoked by the non-ideal rotational movement of the sample grew as well. The undesirable side bands provoked by the Q modulation could be minimized only by improving the mechanical precision of those parts which influence the rotational movement (sample cell, rotor, air turbine). However, here one then arrived at the limits of the degree of precision that could be achieved.
From "Journal of Magnetic Resonance 80, pages 547 to 552, (1988) , there has been known a method for subsequent compensation of equipment errors by deconvolution, by means of an internal reference signal. According to this method, a reference signal is extracted by means of digital filters for generating an idealized spectrum using the before-mentioned deconvolution technique. The measures proposed by this paper are suited also for subsequent elimination of rotational side bands.
While it is relatively easy with the aid of the appliances proposed by the prior art for reducing rotational side bands to achieve side band intensities of about 10% of the main band intensity, it is frequently very difficult to reduce this value further to the desired value of less than 1%. This is true especially for the side band components provoked by Q modulation.
However, it may be assumed that the side-band intensities achieved by the methods of the prior art are already small relative to the main-band intensities.
From German Disclosure Document No. 28 16 225, in particular claim 1, FIG. 3 and column 2 of the specification, lines 25 to 64, there have been known a method and a device by which the side bands of a nuclear resonance spectrum, which are provoked by the rotation of the sample of the nuclear resonance spectrometer, are smeared over a given frequency range. This leads to a broad side-band component with small amplitude, instead of a sharp component with great amplitude. But the integral of the side-band component is equal in both cases. Consequently, the known method does not result in compensation of the rotational side bands; the side bands are not eliminated, but appear as broad-based hill.